A part time four-wheel drive vehicle falls in a four-wheel drive state when a transfer, which is a two-wheel/four-wheel drive switching mechanism, is directed to a transmitting state in order for part of a torque to main driving wheels to be directed to auxiliary driving wheels and in a two-wheel drive state only by means of the main driving wheels when the transfer is directed to a non-transmitting state in order for the torque not to be directed to the auxiliary driving wheels and can perform a two-wheel/four-wheel drive switching between the two-wheel drive and the four-wheel drive according to a state switching between the non-transmitting state and the transmitting state.
Incidentally, during the two-wheel drive in which the transfer is in the non-transmitting state, a transmitting system of the auxiliary driving wheels, which are not presently driven, namely, a power transmission system of the transfer and between the auxiliary driving wheels is in a reverse drive state in which the torque is transmitted from the auxiliary driving wheel side to the transfer side (accurately speaking, an auxiliary driving train is revolved from a tire side (the auxiliary driving wheel side) as opposed to a positive drive state during the four-wheel drive (a state in which the torque is transmitted from the transfer side to the auxiliary driving wheel side). Therefore, a large influence of a revolving force from the auxiliary driving wheel side is given.
A driving resistance generated according to this reverse drive gives more or less an influence on a vehicular vibration, noise, and a fuel consumption. Hence, its countermeasure is needed. That is to say, the auxiliary driving train is revolved from the tire side so that an ill influence of loss and so forth is given. Hence, its countermeasure becomes necessary.In other words, an auxiliary driving wheel side drive shaft, an auxiliary driving wheel side differential, an auxiliary driving wheel side propeller shaft, and an auxiliary driving wheel side clutch element of the transfer (hereinafter, omitted as rotary bodies for simplicity) are mechanically coupled with the auxiliary driving wheels. These rotary bodies are revolved together with one another along with the revolution of auxiliary driving wheels during the two-wheel drive. In addition, during the two-wheel drive, these rotary bodies are revolved in spite of the fact that these rotary bodies are not needed to be revolved. The revolution of these rotary bodies causes a friction resistance in bearings or gears and a rotary inertia to be a driving loss.
To cope with this countermeasure, a two/four-wheel drive switching control apparatus for the part time four-wheel drive vehicle has been proposed for a part time four-wheel drive vehicle, for example, as described in Patent Document 1.
The two/four-wheel drive switching control apparatus for the part time four-wheel drive vehicle described in Patent Document 1 separates one of the left and right auxiliary driving wheels from the auxiliary driving wheel drive train at a time of the two wheel drive from the auxiliary driving wheel drive train so that the separated auxiliary driving wheel is in a free state. Thus, the drive resistance along with the above-described reverse drive is reduced as low as possible and the ill influence on the vehicular vibration, noise, fuel consumption, and so forth is eliminated as remarkably as possible.